Small particles of liquid or solid matter suspended in gases are of increasing importance in science and technology. In the air, such particles are a troublesome byproduct of combustion and other industrial processes, there exists, therefore, a need to remove them as completely as possible. In particular human generated aerosols are used in weather modification methods to act as nuclei for the condensation of ice crystals. Another example of use is in the production of smoked meat, where the particles are produced in a smoldering fire and have to be deposited onto the meat.
If the particles are charged electrically, it is possible:
(i) to remove them from the gas, PA1 (ii) to measure their number and size distribution, PA1 (iii) to deposit them at any desired location, PA1 (iV) to influence their coagulation. PA1 (1) operates for a range of particle sizes including very small particles, PA1 (2) depends on the particle material and particularly on the chemicals adsorbed at the surface, PA1 (3) is non-destructive, PA1 (4) may be simple and inexpensive.
Such electrical charging has been achieved so far by first producing ions of positive or negative electrical charge in the gas via an electrical discharge, a radioactive source or other means, and letting the ions attach to the particles by diffusion. This diffusion charging is non-destructive, but it has the disadvantage of being very inefficient for ultrafine particles. In air for instance, particles with a diameter of less than 10.sup.-8 m cannot be charged with appreciable efficiency. Furthermore, diffusion charging depends only on the size of the particle and not on its chemical properties or the state of its surface. In nucleation, and in the catalysis of chemical reactions, it is the small particles and the adsorbate condition on their surface that is of crucial importance. Therefore, there is a need for a charging mechanism that allows charging of very small particles, and additionally distinguishes between particles of different chemical composition and surface properties. Also, the charging should be non-destructive and leave the particle surface and environment unchanged.